1. Field of the Invention
This invention relates to an optical disk apparatus and a focal position control method and a focal position control apparatus that can suitably be applied to an optical disk apparatus for recording on and reproducing data from an optical disk having a plurality of recording layers.
2. Description of the Related Art
Blu-ray disks (Trademark of Sony Corporation) that are large memory capacity optical disks realized by using a violet laser of a short wavelength and an objective lens having a high numerical aperture have become commercially available in recent years. Single layer Blu-ray disks having only a single recording layer (BD-SL: Blu-ray Disk Single Layer) and dual layer Blu-ray disks having two recording layers arranged along the height of the disk (BD-DL: Blu-ray Disk Dual-Layer) have been standardized.
Thus, an optical disk apparatus for recording signals to and reproducing signals from a Blu-ray disk is required to reliably focus the optical beam on the target recording layer of the optical disk and pull in the focus servo when the optical disk is mounted in the optical disk apparatus.
When the focus servo slips off due to an external turbulence or the like during an operation of recording signals on or reproducing signal from an optical disk, it is necessary for the optical disk apparatus to quickly restore the position (address) where it was accessing immediately before the external turbulence, which may be an impact. Then, again the optical disk apparatus is required to reliably focus the optical beam on the target recording layer and pull in the focus servo.
As for controlling pulling in the focus servo for a dual layer Blu-ray disk (to be referred to as focus search hereinafter), methods have been proposed to detect the surface of the cover layer of the optical disk according to the change in the level of the sum signal or the focus error signal output from the optical pickup of the optical disk apparatus (see, for example, Jpn. Pat. Appln. Laid-Open Publication No. 2004-39125).
The sum signal is the sum of the detection signals output from the photodetector of the optical pickup and its signal level varies as a function of the quantity of light reflected from the optical disk. Therefore, the signal level rises when the focus of the light beam comes close to the surface of the cover layer or the recording layer, which is a reflector of light. The focus error signal is, on the other hand, proportional to the distance from the focal point of the light beam to the recording layer. It is adapted to cover a certain distance from the in focus position for outputting focus error signals. Thus, the focus error signal draws a substantially S-shaped curve when the focal point of the light beam passes the recording layer.
Therefore, referring to FIG. 1 of the accompanying drawings, it is so defined that the surface of the cover layer is detected when the first increase of the sum signal is detected as the objective lens is driven to move closer to the objective lens, while monitoring the sum signal and the focus error signal. Additionally, it is so defined that the focal point of the light beam comes close to the recording layer when the objective lens falls in a state where the signal level of the sum signal is found not lower than a predetermined sum signal threshold value and a rise of the level of the focus error signal is detected as the objective lens is driven to move further to approach the optical disk.
As a matter of course, a rise of the level of the focus error signal occurs once when the optical disk has a single recording layer and twice when the optical disk has two recording layers. When a focus search operation is conducted for the recording layer of a single layer disk or the first recording layer of a dual layer disk, the first rise of the level of the focus error signal is determined to be a hit at the target recording layer.
When, on the other hand, a focus search operation is conducted for the second recording layer of a dual layer disk, the second rise of the level of the focus error signal that takes place after a fall of the level of the sum signal to below the above described threshold value (detection of a fall of the level of the sum signal) and the following second rise to the above threshold value for the level of the sum signal is determined to be a hit at the target recording layer.
Alternatively, when a focus search operation is conducted for the second recording layer of a dual layer disk, the detection of the first rise of the level of the focus error signal after detecting the second rise (L0 layer) of the level of the focus error signal and reversing the direction of driving the object lens so as to move it away (return) from the optical disk after driving the objective lens to move closer to the optical disk and make the focal point thereof overrun the first layer (L0 layer) is determined to be a hit at the target recording layer.